1. Field of the Invention
The present invention relates to a circuit for measuring the inter-pulse time difference between a first pulse and a second pulse and more particularly to a circuit which can be effectively used in an apparatus which measures the distance to a target based on the delay time between the emission of a pulsed laser light and the reception of its reflected light.
2. Description of the Related Art
In the past, as shown for example in JP-A-59-203975, there has been proposed a vehicle optical radar system of the type in which a beam-like light signal is transmitted substantially parallel to a road surface to detect the distance to an object based on the propagation delay time which is required for receiving the reflected light signal from the object, and the system has been so designed that the direction of travel of the beam-like light signal is rotated parallel to the road surface so as to extend the maximum detection distance and to prevent the missing of a preceding vehicle at a curve in a road.
In the vehicle optical radar system of the above type, a distance detecting circuit includes an R-S flip-flop which is set by a beam-like light signal or transmitted signal and which is reset by a received signal, a high-frequency oscillator for generating a high-frequency pulse train, an AND gate for receiving the output of the R-S flip-flop and the output of the high-frequency oscillator, and a high-speed counter for counting the number of pulses in the high-frequency pulse train supplied through the AND gate. Then, in response to the output from the R-S flip-flop, the AND gate is opened for the period of the time difference between the transmitted signal and the received signal (the porpagation delay time) thereby supplying the high-frequency pulse train to the high-speed counter, whereby the high-speed counter generates calculated value of the distance corresponding to the time difference.
Then, where the distance is computed from the propergation delay time of the beam-like light signal as in the case of the above-mentioned conventional circuit, the following relation holds: EQU T&gt;2L/C(C:light velocity)
where T represents the propagation delay time and L represents the distance. From the above equation, the propagation delay time T per meter of the distance L becomes 6.67 ns so that in order to obtain a resolution of 1 m for the detection distance, the oscillation frequency of the high-frequency oscilaltor must be set to 150 MHz. Also, since the high-speed counter must measure such an extremely high frequency, it is necessary to use a counter including an expensive ECL (emitter coupled logic) or the like. Further, while the resolution of the detected distance can be enhanced further by simply increasing the oscilaltion frequency, it is impossible to increase the oscillation frequency infinitely and therefore there is naturally a limitation to the resolution of the detected distance.
For instance, when constructing a physical quantity measuring apparatus in which a physical quantity e.g., a distance is converted to a corresponding time and the physical quantity is measured in accordance with the converted time, it is desired to construct an inter-pulse time difference measuring circuit designed so that the time corresponding to a physical quantity is expanded by a given multiplication factor and the physical quantity is computed from the expanded time, thereby obtaining an arbitrary detection resolution.